Both Telstra / Telechoice (probably other postpaid telstra MNVOs as well).
So my partner went over her allocated 1GB cap on Telechoice so I went looking through the data session records to find that about 300MB was from the previous month, and for the month in question if you take off that data that would make her under.
I looked back and it was commonly happening with both of our accounts - some of the data sessions from the previous month were being added to the next months bill. I remember this happening once years ago when I was on Telstra itself.
This is wrong - a customer expects to be able to fully use their allocated data allowance within the billing cycle. I can't believe that Telstra thinks that it's good enough to bill like this.
Telechoice refunded the excess but I'm still very uncomfortable that Telechoice's (and probably Telstra's) billing systems are wrong and there would probably be thousands of customers with excess usage charges that aren't legitimate.
A billing month doesnt start on the 1st of the month, it starts on the anniversary date of the bill. So if you start paying on the 12th, then data resets on that date each month. They usually pro rata that as well. Or they might have rolling usage, which is rare. IE the system doesnt update immediately, and data carries over to the next month.